Computer Networking CNS Chapter 2 Quiz

Computer Networking and IT Security (CNS)

• Course: INHH0012
• Semester: WiSe 2024/25
• Instructor: Prof. Dr.-Ing. Stephan Günther
• Institution: Technical University of Munich (TUM)
• Date: Saturday 2nd November, 2024

Chapter 2: Data Link Layer

• Focuses on Local Area Networks (LANs)
• Characterizes connections, multiple access, and access control
• Includes framing, addressing, and error detection
• Covers connecting nodes on Layers 1 and 2
• Discusses security considerations
• Presents a summary of the chapter content

Chapter 2: Data Link Layer: Representation of Networks as Graphs

• Uses graphs to represent networks
• Includes directed graphs and undirected graphs
• Explains paths in networks and network topologies
• Introduces adjacency and distance matrices
• Explains how to create tree structures

Directed Graphs

• Commonly used to represent network topologies and node connections
• An asymmetric network can be represented as a directed graph G = (N, A)
• N: Set of nodes/vertices
• A: Set of directed arcs {i, j} | i, j ∈ N ∧ i, j are connected by a directed arc}
• Example: N = {1, 2, 3, 4}, A = {(1, 2), (2, 3), (2, 4), (1, 4)}

Undirected Graphs

• Represents a symmetric network as an undirected graph G = (N, E)
• N: Set of nodes
• E: Set of (undirected) edges {i, j} | i, j ∈ N ∧ i, j are connected undirectedly}
• Example: N = {1, 2, 3, 4}, E = {{1, 2}, {2, 3}, {2, 4}, {1, 4}}

Paths in Networks

• A path between two nodes s, t ∈ N is a set Pst = {(s, i), (i, j),...,(k, l),(l, t)} of edges connecting s and t
• Path cost: Sum of costs of used edges: c(Pst) = ΣCij (i,j)∈Pst
• Path length: Number of intermediate nodes: I(Pst) = |Pst|
• Layer 3 path costs are often referred to as hop counts which are less common on Layer 2.
• Source and destination are commonly denoted as 's' and 't' respectively

Network Topologies

• Descriptions of how nodes are interconnected in a network
• Well-known Topologies: Point-to-point, Chain, Star, Mesh, Tree, Bus

Adjacency and Distance Matrix

• Adjacency matrix (A) represents connections between nodes in a network
  • A(i, j) = 1 if there's a connection from node i to node j, otherwise 0
  • Symmetric if A = AT
• Distance matrix (D) shows the shortest path cost between all node pairs.
  • D(i, j) = cost of the shortest path of length 1 between nodes i and j
  • D(i, i)= 0, dij= ∞ if no direct connection

Question and Answer About Adjacency and Distance Matrix

• Question: How do we get a matrix of the shortest path costs between any pair of nodes?
• Answer: Calculate powers of D with respect to the min-plus product
  • (Dⁿ with d = min {d^i-1 + d_kj} for i, j, k ∈ N)
  • The nth power (Dⁿ) contains costs of shortest paths of length n hops or less.

Creating Tree Structures

• Trees are connected, loop-free graphs
• Two special types are discussed: Shortest Path Tree (SPT) and Minimum Spanning Tree (MST)
• SPT connects a root to all other nodes with minimum total cost.
• MST connects all nodes with minimum total cost for the needed arcs (unrooted tree).

Characterizing Connections, Media Access, ALOHA, CSMA, CSMA/CD, CSMA/CA, Token Passing

• Data rate, transmission delay, direction of transmission, and multiple access
• Different media access control methods:
  • ALOHA (early wireless networking protocol)
  • Slotted ALOHA
  • CSMA/CSMA/CD
  • CSMA/CA
  • Token Passing

Addressing and Error Detection

• Unique node identification
• Broadcast and multicast addresses
• MAC addresses (Media Access Control)
• Error detection codes (e.g., checksums/cyclic redundancy checks (CRC)) are used to increase reliability)
• Handling of frame boundaries and code transparency (essential for reliable transmission and receiver recognition)

Security Considerations (Layer 2)

• CAM (Content Addressable Memory) poisoning, an attack where an attacker can forge MAC addresses and disrupt network communications.
• Denial of Service (DoS) attack is performed by flooding the switch with a large number of requests from arbitrary source addresses
• Mitigations for layer 2 attacks (e.g. Allowed source address, dynamic port security, automatic removal of obsolete entries)
• Cable and wireless security considerations (limiting access to the medium, authentication, wireless security standards, like WEP, WPA, WPA2, and WPA3)